Semiconductor laser modules (which have a semiconductor laser, etc.) are usually employed as signal light sources for optical fiber communications, particularly trunk lines and cable television (CATV), or as pumping sources for optical fiber pumps. In order to realize high output and stable operation, such semiconductor laser modules have a semiconductor laser, a photodiode chip, optics such as a lens, a thermistor, etc., which are arranged at predetermined positions on a metal plate mounted on a thermo-control module capable of controlling temperature in dependence on the magnitude and direction of current conduction.
FIG. 8 shows a conventional semiconductor laser module having a thermo-control module incorporated therein. As shown in the figure, the semiconductor laser module 101 has a thermo-control module 103 mounted on the bottom plate 102 of the package through a lower joining solder portion 110. The thermo-control module 103 has a metal plate 105 mounted thereon. The metal plate 105 is provided with a carrier substrate 106, a semiconductor laser 107, a condensing lens 108, etc. The lower substrate 104a of thermo-control module 103 and the package bottom plate 102 are joined with the lower joining solder portion 110. The upper substrate 104b of the thermo-control module 103 and the metal plate 105 are joined with an upper joining solder portion 111. In the conventional semiconductor laser module, the lower substrate 104a of the thermo-control module 103 and the bottom plate 102 of the package are joined with solder consisting of an alloy of 63 wt % (weight-percent) Sn and 37 wt % Pb, and the lower joining solder portion 110 is formed. As shown in FIG. 1C, there is also a semiconductor laser module 1 having no internal thermo-control module.
However, the conventional optical module has the following disadvantages. That is, when joining the thermo-control module 103 and the package bottom plate 102 with solder, load is applied while melting the solder, but since the way of applying this load is not constant, the joining solder portion 110 is not uniform in thickness, as shown in FIG. 8. If the joining solder portion 110 is not uniform in thickness, it will be deformed due to a change in the temperature of the environment where the optical module is used, or a change in temperature when manufacturing the optical module. This deformation causes, for example, positional misalignment between the optical component 108 and optical fiber 109 optically aligned through the joining solder portion 110. As shown by an alternate long and short dash line, an error occurs in the axial alignment between the optical component 108 and the optical fiber 109 and causes a reduction in a coupling efficiency with an optical fiber and degradation of light output.
In addition, in a thin portion of the joining solder portion 110 (indicated by reference numeral 112), cracks are caused to occur in the joining solder portion 110 by thermal stress resulting from a difference in thermal expansion rate between joined members (e.g., the lower substrate 104a and bottom plate 102 in FIG. 8) during a change in temperature. As a result, a reduction in the joining strength and a change in the light coupling efficiency are caused. Furthermore, it is preferable to use lead-free solder that is environment-friendly.
The above-described conventional optical module is shown in Japanese Laid-Open Publication Nos. 2000-323731, 2000-280090, Hei 7-128550, and Hei 11-295560.